This invention relates to d.c.-to-d.c. converters in general and, in particular, to a d.c.-to-d.c. converter designed for zero-voltage switching of a main switch; that is, the main switch of the converter is turned on and off when the voltage across the same is approximately zero for minimal power loss.
U.S. Pat. No. 5,719,755 to Usui is hereby cited as describing and claiming a flyback d.c.-to-d.c. converter bearing particular pertinence to the instant invention. This prior art converter has a transformer with a primary winding connected across a direct-current power supply via an on-off switch, and a secondary winding connected across a load via a rectifying and smoothing circuit. A capacitor is connected in parallel with the switch for partial resonance.
The fundamental operating principle of the flyback d.c.-to-d.c. converter is such that the transformer stores energy from the power supply when the switch is closed, and releases the stored energy for powering the load when the switch is open. Zero-voltage switching is automatically accomplished when the switch goes off, because there is no voltage across the switch when it is on. The voltage across the capacitor rises from zero during each nonconducting period of the switch.
Difficulties were experienced, however, in zero-voltage turning-on of the switch. Should the resonant capacitor have some charge left thereon when the switch was turned on, that charge would be released through the switch, resulting in power loss. It was suggested and practiced to lessen this power loss by causing the capacitor to complete discharge before the switch was turned on.
In a typical conventional zero-voltage-switching method, after the energy that had been stored on the transformer during each conducting period of the switch was released during the ensuing nonconducting period of the switch, the resonant capacitor was discharged by the resonance of the transformer primary and the capacitor. The switch was turned on when the voltage across the resonant capacitor, and hence across the switch, became practically zero. Zero-voltage turning-on of the switch was thus accomplished, but under limited conditions.
The above conventional solution proved unsatisfactory in cases where the input voltage varied much as, say, from 100 to 230 volts. The conducting period of the switch grew less with an increase in input voltage under these conditions. Less energy was stored on the transformer during such shorter periods of time, and correspondingly less time was required for its discharge. The result was the flow of an oscillatory current through the resonance circuit of the capacitor and transformer primary following the completion of discharge.
For this reason the charge on the capacitor was not necessarily been zero when the switch was turned on; in other words, zero-voltage switching did not take place. The efficiency of the converter deteriorated in the cases noted above, as well as in the event of a great reduction in the power requirement of the load.
The present invention aims, in a d.c.-to-d.c. converter of the kind defined, at zero-voltage switching of the main switch when the switch is not only tuned on but off as well, totally independently of how long the switch is held turned on.
Briefly, the present invention may be summarized as a zerovoltage-switching d.c.-to-d.c. converter to be connected between a d.c. power supply and a load, comprising a transformer having a primary, a secondary, a tertiary, and a quaternary winding. The transformer primary is connected via a first switch to a pair of input terminals which are to be coupled to a d.c. power supply, the first switch being connected in parallel with a resonant capacitor or like capacitance means. The transformer secondary is connected via a rectifying and smoothing circuit to a pair of output terminals which are to be connected to a load to be powered. The transformer tertiary and quaternary are connected in series with each other and with resonant inductance means and a first diode and a second switch, and in parallel with the serial connection of the transformer primary and the first switch. A second diode is connected in parallel with the serial connection of the transformer quaternary and the resonant inductance means and the first diode and the second switch. Also included is a switch control circuit connected to the first and the second switch for making on-off control of these switches. The switch control circuit includes means for turning on the second switch at a first moment that is earlier than the starting moment of each conducting period of the first switch and turning off the second switch at a second moment that is equal to or earlier than the ending moment of each conducting period of the first switch.
The second switch, newly introduced by the instant invention, serves the purpose of compulsorily discharging the resonant capacitor and hence making zero the voltage across the first switch. Thus is accomplished the zero-voltage switching of the first switch when the same is turned on, in addition to when it is turned off.
The first and second switches are controlled by the common switch control circuit in prescribed time relationship to each other. Despite changes in the conducting periods of the first switch, the resonant capacitor is forcibly discharged by the second switch to enable zero-voltage turning-on of the first switch. The switch control circuit requires addition of a minimal number of parts to the preexisting ones for controlling the first switch.
The second switch is itself well calculated not to adversely affect the efficiency of the converter. The second switch is turned on at zero current, and off at zero voltage.
The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference had to the attached drawings showing some preferred embodiments of the invention.